1. Field of the Invention
The present invention relates to a wavelength detector, a wavelength stabilization laser device, and an image display device, and particularly to a wavelength detector capable of distinguishing between the changes of a transverse mode and of a longitudinal mode of a laser light and thus detecting the change of the wavelength of the laser light, to a wavelength stabilization laser device using the wavelength detector, and to an image display device using the laser light generated by the wavelength stabilization laser device as a light source.
2. Description of the Related Art
In recent years, an image display device using laser light sources attracts attention. Since a laser light is a coherent light, the width (spectral width) of the wavelength of the laser light is extremely narrow. Since the wavelength determines a color, the laser light has excellent monochromaticity. The wavelengths of the laser light sources may be appropriately selected, whereby it is possible to realize an image display device having high color purity of three primary colors of light: R (red), G (green), and B (blue), and also having high color reproducibility. When the wavelengths change, the color of an image changes naturally. Therefore, a method for detecting the changes of the wavelengths and a method for stabilizing the wavelengths are very important in the image display device using the laser light sources.
Next, a conventional wavelength detector using a diffracted light will be described. FIG. 21 is a schematic diagram showing an example of a conventional wavelength detector 114 using a diffracted light. Referring to FIG. 21, the wavelength detector 114 includes a diffraction grating 111 for diffracting a laser light emerging from a laser light source 110, and also includes a photodetector 130 for measuring a +1-order diffracted light. The laser light outputted from the laser light source 110 is incident on the wavelength detector 114 provided in the light path of the laser light. In the wavelength detector 114, the laser light incident thereon is diffracted by the diffraction grating 111, and the diffracted laser lights interfere with one another and thus generate In-order diffracted lights 120. A light incidence surface of the photodetector 130 for measuring the +1-order diffracted light is designed to be equal to or smaller than a beam spot of the laser light in size.
The photodetector 130 is mechanically movable so as to move to a position at which the measured value of the light intensity of the +1-order diffracted light is great. Here, the position of the photodetector 130, at which the measured value of the light intensity of the +1-order diffracted light is great, is a position at which the beam spot is included within the light incidence surface of the photodetector 130. The wavelength detector 114 can recognize the position of the beam spot of the +1-order diffracted light by moving the photodetector 130 while observing the measured value of the light intensity of the +1-order diffracted light. Thus, based on the position of the beam spot, it is possible to obtain a diffraction angle θ1 of the +1-order diffracted light with respect to a 0-order diffracted light in a fundamental mode. Since the diffraction angle θ1 of the +1-order diffracted light with respect to the 0-order diffracted light in the fundamental mode corresponds to the wavelength of the laser light, the wavelength detector 114 can detect the change of the wavelength of the laser light based on the change of the diffraction angle θ1 of the +1-order diffracted light.
Note that the conventional wavelength detector 114 can similarly detect the change of the wavelength of the laser light also by fixing the photodetector 130 and moving the diffraction grating 111. Further, the conventional wavelength detector 114 can similarly detect the change of the wavelength of the laser light also by fixing the photodetector 130 and dividing the light incidence surface of the photodetector 130 into an array. In this case, the wavelength detector 114 calculates the diffraction angle θ1 of the +1-order diffracted light based on the values of the light intensities measured separately by all of the divided areas of the light incidence surface, whereby the wavelength detector 114 can detect the change of the wavelength of the laser light based on the calculated diffraction angle θ1.
Further, the following publications each disclose a semiconductor laser light source applying the method of the conventional wavelength detector 114. A semiconductor laser light source disclosed in Japanese Examined Patent Publication No. 62-005677 (hereinafter referred to as Patent Document 1) detects the change of the wavelength of a laser light by causing a position sensitive detector to detect a +2 diffracted light diffracted by a diffraction grating and thus obtaining the diffraction angle of the +2 diffracted light. Additionally, the semiconductor laser light source disclosed in Patent Document 1 measures laser power by causing a photodetector to detect a 0-order diffracted light and feeds a +1-order diffracted light back to a semiconductor laser. The semiconductor laser light source disclosed in Patent Document 1 concurrently measures the change of the wavelength of the laser light and the laser power, thereby realizing a stable operation in which the change of the wavelength is small.
Further, a semiconductor laser light source disclosed in Japanese Laid-Open Patent Publication No. 2006-32397 (hereinafter referred to as Patent Document 2) detects the change of the wavelength of a laser light by causing a position sensitive detector to measure a +1-order diffracted light and thus obtain the diffraction angle of the +1-order diffracted light. Then, the semiconductor laser light source disclosed in Patent Document 2 controls laser power based on the change of the wavelength of the laser light, thereby realizing a stable operation in which the change of the wavelength is small.
Further, a semiconductor laser light source disclosed in Japanese Laid-Open Patent Publication No. 2006-324371 (hereinafter referred to as Patent Document 3) inserts, taking advantage of an oval beam shape of a laser light emerging from a semiconductor laser, an optical device for reducing a beam diameter in a longitudinal direction, when causing a position sensitive detector to measure a +1-order diffracted light, thereby improving the accuracy of detecting the change of the wavelength of the laser light.
However, a laser light has two types of changes of modes: the change of a longitudinal mode and the change of a transverse mode. The change of the longitudinal mode is the change of a wavelength. That is, detecting a wavelength is measuring the state of the longitudinal mode. On the other hand, the change of the transverse mode is the change of the intensity distribution of beam cross-sections. The above-described wavelength detector 114 and the semiconductor laser light sources disclosed in Patent Documents 1 through 3 each merely measure the state of the longitudinal mode of the laser light without taking the change of the transverse mode into account.
Generally, when the transverse mode is in a fundamental mode (a single transverse mode), the intensity distribution of beam cross-sections is a Gaussian distribution (a normal distribution). When the transverse mode changes, the Gaussian distribution changes and the positions of the beam spots of the laser lights change. That is, since none of the conventional wavelength detector 114 and the semiconductor laser light sources disclosed in Patent Documents 1 through 3 take the change of the transverse mode into account, an error may be caused in wavelength detection when the transverse mode changes.
With reference to the conventional wavelength detector 114, the above-described wavelength detection error caused in the case where the transverse mode changes will be described. First, it is assumed that as shown in FIG. 21, the +1-order diffracted light of the diffraction angle θ1 is generated in the conventional wavelength detector 114. FIG. 22 shows a state where the transverse mode is constant and the longitudinal mode changes from the above-described state. As shown in FIG. 22, when the longitudinal mode changes, the diffraction angle of the +1-order diffracted light changes naturally, and thus θ1+Δθ. As a result, the wavelength detector 114 moves the photodetector 130 upward so as to move the light incidence surface of the photodetector 130 to the position of the beam spot of the +1-order diffracted light.
Next, FIG. 23 shows a state where the longitudinal mode is constant and the transverse mode changes. As shown in FIG. 23, as a result of the change of the transverse mode, the position of the beam spot shifts upward. That is, since the +1-order diffracted light shifts upward naturally, the light incidence surface of the photodetector 130 shifts upward. The position of the photodetector 130 in this case is the same as the position of the photodetector 130 measuring the +1-order diffracted light in FIG. 22. Consequently, the wavelength detector 114 cannot distinguish between the state where the longitudinal mode changes (FIG. 22) and the state where the transverse mode changes (FIG. 23). That is, the conventional wavelength detector 114 recognizes even the change of the transverse mode as the change of the longitudinal mode, and thus an error may be caused in wavelength detection of the laser light.
Further, when the image display device uses the laser light sources, it is required to improve the efficiency of the laser light and the output of the laser light to realize the image display device having a higher brightness. However, the greater the output of the laser light, the more likely the changes of the modes of the laser light occur. As an example, when the laser light sources are used as a backlight of a 37-inch liquid crystal television, each color of R, G, and B requires a more-than-3W laser light source in view of efficiency and the like. To realize a sharper image regardless of the changes of the modes, a high-accuracy wavelength detector is necessary.
Further, in the laser light sources used for the image display device, a laser light is scanned optically or mechanically and thus expanded to be projected onto a screen. Furthermore, to realize a liquid crystal backlight using a planar lighting device having laser light sources, a method is known for expanding a laser light and thus making the laser light incident on a light guide plate. To improve the accuracy of wavelength detection of the laser light expanded in the above-described manner, general methods may include one for providing an aperture to reduce the effect of the expansion of the laser light, another for using a lens to concentrate the laser light into a narrow beam and thus measuring the narrow beam, and the like. However, when the aperture is provided, the light intensity of the laser light to be measured is reduced. Moreover, the method for using the lens to converge the laser light requires an optical system provided in accordance with the state of the laser light, and thus complicates the device.